The present invention relates to a sensor for analyzing a stream of exhaust gas components, the sensor including a limit current measurer, and a method for determining exhaust gas components, in particular, by using the sensor.
A plurality of instruments for the analysis of exhaust gases of motor vehicles are described, for example, in the book xe2x80x9cAutomotive Electronics Handbookxe2x80x9d (1995), McGraw Hill Inc., Section 6 xe2x80x9cExhaust Gas Sensors.xe2x80x9d Such instruments include, for example, the xcex=1 probe, which is an equilibrium sensor that checks, by measuring the Nernst voltage, whether the air-fuel mixture injected into a gasoline engine has a xcex value of approximately one. The UEGO sensor (also known as the universal sensor) is also an equilibrium sensor, which is operated as a combination of a sensor based on the Nernst principle and a limit value probe, which are immersed in the exhaust gas of the internal combustion engine and whose measuring current, which depends on the xcex value, is used to regulate the xcex value. The operation of mixed-potential sensors, which are disequilibrium sensor type instruments, is based on the fact that reduced catalytic activity prevents a gas equilibrium from being established on the electrode of a ZrO2 galvanic cell. As a result, no state of oxidation/reduction equilibrium can be established in oxygen and a mixed potential is formed, which is determined, among other things, by electrode activity, temperature, and gas composition. By xe2x80x9cpassivelyxe2x80x9d measuring a signal dependent on the state of the electrode, mixed-potential sensors allow conclusions to be drawn concerning the gases in question. Their use in practice, however, is problematic, since they only operate properly in a very narrow temperature range, and their signal is often dependent on their historyxe2x80x94their properties change as they age. A NOx pump sensor is also a disequilibrium sensor. It is used for determining NOx in the presence of oxygen. It operates as follows: oxygen is pumped out of a first cathodic limit current cell, the electrode being made of platinum-gold, which prevents NOx from also being pumped out. Therefore, a limit current, which is proportional to the NOx level in the exhaust gas, can be measured in a second cathodic limit current cell.
To date, there is no known field-usable method of determining the oxidizable components of exhaust gases in the presence of the reducible components with the accuracy required by law.
An object of the present invention is to provide such a method.
The sensor according to the present invention is a disequilibrium sensor having a simple design. Reducible and oxidizable gases can be analyzed by this single sensor, i.e., no separate sensors are needed for the two analyses. Analysis is performed using current limit probes. Current limit probes xe2x80x9cactivelyxe2x80x9d measure the diffusion characteristics. Their electrodes only have to pump and prevent catalysis or only allow anodic oxidation. Aging may necessitate a slight increase in the pump voltage to reach the limit current. The measured signal, however, is actually the diffusion resistance. The sensor according to the present invention contains no closed-circuit control, so no expensive electronic circuitry is needed. The sensor is well suited for measuring exhaust gases both in engines operated in the lean range, such as diesel engines, and in engines, such as gasoline engines, operated in the xcex=1 range. The sensor according to the present invention can determine the sum of both reducible and oxidizable gases with considerable accuracy, so that it can often replace expensive, complex, and bulky analyzers. The sensor can also be used for on-board diagnostics (OBD). The operation of the sensor is based on the fact that, by suitably selecting the electrode material in the cathode cell, its catalytic activity is so low that, despite the very high temperatures, a reaction between reducible and oxidizable gases is almost impossible even if there is an excess of reducible exhaust gas components such as oxygen. The lowest possible catalytic activity of the electrode material can also be supported by appropriate material morphology, the most favorable morphology being determined by simple tests.
The electrode material of the cathode cell is advantageously made of platinum-gold.
The two limit current pumps can be advantageously mounted on a single substrate. This not only makes a very compact sensor arrangement possible, but also results in a sensor so similar to the UEGO sensor, that it can be manufactured on the UEGO sensor assembly line without considerable adjustments being required.
The limit current pump for oxidizable gases and the limit current pump for reducible gases can be advantageously operated at a constant pump voltage. The limit current pump for reducible gases can, however, also be operated at a pump voltage that is independent of the limit current to avoid decomposition of H2O and CO2 in the exhaust gas.
In an advantageous embodiment of the sensor according to the present invention, at least two selective pump cells are provided for oxidizable gases, the electrode materials being selected so that in the cells upstream from the last cell in the direction of diffusion only the reaction of one oxidizable exhaust gas component is allowed. The materials are selected for their composition, with spinel advantageously added, for example, and for their morphology.
The oxidizable components of a lean exhaust gas and the reducible and oxidizable components of an exhaust gas with xcex=1 can be advantageously determined using the method according to the present invention. This application is particularly advantageous, since it allows the efficiency of a three-way catalyst to be determined in the range from 0% to 100%. The A probes used so far only measure in the approximate range of 80% to 100%.
With the embodiment of the sensor according to the present invention having at least two pump cells for oxidizable gases, the carbon monoxide and ammonia or carbon monoxide and hydrocarbon levels can be advantageously determined side by side. Selectivity, for example, in the SCR method (see below), and detection of NH3 in the presence of CO in the exhaust gas can be achieved by suitable actions on the system such as an upstream oxidation catalyst to oxidize CO prior to introducing NH3 or its precursor.